Polymer membranes are often used commercially as a means of achieving gas separations on a relatively small or intermediate scale. Such membranes can offer an advantage in terms of energy consumption and lower capital investment in comparison to other gas separation technologies, such as cryogenic distillation. However, currently available gas separation membranes can be problematic with respect to long term stability, particularly for applications in which water is present, due to hydrolytic instability. In addition, the stability of gas separation membranes can be affected by the presence of other components that may cause swelling of the membrane, and consequently cause loss of performance.
In addition, currently available membrane compositions suffer from a well-known trade-off of permeability versus selectivity, wherein highly permeable polymers have limited selectivity for separating the gas-phase components of interest. The trade-off between the permeability and selectivity of gas separation membranes has been extensively reviewed by Robeson in his articles describing the “upper bound” in polymeric gas separation membranes (J. Membrane Sci., 1991, 62: 165-185; J. Membrane Sci., 2008, 320: 390-400). Much effort has been expended to devise polymers that fall above this “upper bound” limit, but with minimal success.
The crosslinking of membrane materials by ultraviolet (UV) irradiation for gas separations was disclosed by Hayes (U.S. Pat. No. 4,717,393). Hayes reported that the UV treatment of polyimides improved separation performance while diminishing the propensity of polyimide systems to hydrolyze, although no data on the latter property were presented. Hayes also did not report any mechanical properties of membrane materials, such as stress/strain behavior at room temperature. Thus, there was no indication that the materials cited would be sufficiently durable for use in hollow fiber or asymmetric membrane systems. In addition, Guillet has provided an extensive review of polymer photo processes (Polymer Photophysics and Photochemistry: An Introduction to the Study of Photoprocesses in Macromolecues, 1987, Cambridge University Press).
Others have sought to apply Hayes' results in polyimide systems to other materials, but the irradiation of other types of membranes to improve gas separation performance or other physical properties has been unsuccessful. Wright and Paul reported the irradiation of polyarylate copolymers, and showed slight increases in selectivity for gas separations upon irradiation (J. Membrane Sci., 1997, 124: 161-174). However, the presence of ester groups in those systems led to competing chemistries and was problematic with regard to hydrolytic stability in membranes. In an attempt to overcome these deficiencies, Wright and Paul incorporated benzocyclobutene-based monomers into polyarylate systems for purposes of inducing thermal crosslinking. However, this approach required specialized, expensive monomers, and resulted in membranes with gas separation performance inferior to the polyarylate systems (J. Membrane Sci., 1997, 129: 47-53). In addition, the olefin groups introduced for purposes of crosslinking also introduced sites that could lead to undesired thermally-induced reactions during the processing steps required to form thin film membranes or hollow-fiber structures that would subsequently be cross-linked.
Likewise, Zhong has reported the crosslinking of sulfonated polyarlyene ether ether ketone (SPEEK) polymers by incorporating allyl groups into a side chain and using a blended photo-initiator in the polymer film to effect the crosslinking reaction upon irradiation (J. Power Sources, 2007, 164: 65-72). However, as with Wright and Paul's system, a specially prepared monomer was required, and subsequent processing was also required to remove the remaining photo-initiator.
Ishikawa et al. have described the use of crosslinkable polyarylene ethers for fuel cell membranes (U.S. Pat. No. 7,345,135). However, the polymers described in Ishikawa are directed to optimal proton exchange across a membrane. Ishikawa does not describe polymers useful for gas separation membranes.
Thus, there is a continuing need in the art for a gas separation membrane with excellent separation performance and high durability, particularly membranes comprising relatively inexpensive crosslinkable polymers. The present invention addresses this unmet need in the art.